This invention relates generally to the production of extreme ultraviolet and soft x-rays with an electric discharge source for projection lithography. In particular, the invention is directed to a capillary discharge source where the electrodes are encased with dielectric or electrically insulating material so that the electrodes are shielded from the plasma. Additionally, the radiation escapes or is collected through the side of the device or transversely via one or more of a radiation exit, and away from any contact with the electrodes.
The present state-of-the-art for Very Large Scale Integration (xe2x80x9cVLSIxe2x80x9d) involves chips with circuitry built to design rules of 0.25 xcexcm. Effort directed to further miniaturization takes the initial form of more fully utilizing the resolution capability of presently-used ultraviolet (xe2x80x9cUVxe2x80x9d) delineating radiation. xe2x80x9cDeep UVxe2x80x9d (wavelength range of xcex=0.3 xcexcm to 0.1 xcexcm), with techniques such as phase masking, off-axis illumination, and step-and-repeat may permit design rules (minimum feature or space dimension) of 0.18 xcexcm or slightly smaller.
To achieve still smaller design rules, a different form of delineating radiation is required to avoid wavelength-related resolution limits. One research path is to utilize electron or other charged-particle radiation. Use of electromagnetic radiation for this purpose will require x-ray wavelengths. Various x-ray radiation sources are under consideration. One source, the electron storage ring synchrotron, has been used for many years and is at an advanced stage of development. Synchrotrons are particularly promising sources of x-rays for lithography because they provide very stable and defined sources of x-rays, however, synchrotrons are massive and expensive to construct. They are cost effective only when serving several steppers.
Another source is the laser plasma source (LPS), which depends upon a high power, pulsed laser (e.g., a yttrium aluminum garnet (xe2x80x9cYAGxe2x80x9d) laser), or an excimer laser, delivering 500 to 1,000 watts of power to a 50 xcexcm to 250 xcexcm spot, thereby heating a source material to, for example, 250,000xc2x0 C., to emit x-ray radiation from the resulting plasma. LPS is compact, and may be dedicated to a single production line (so that malfunction does not close down the entire plant). The plasma is produced by a high-power, pulsed laser that is focused on a metal surface or in a gas jet. (See, Kubiak et al., U.S. Pat. No. 5,577,092 for a LPS design.)
Discharge plasma sources have been proposed for photolithography. Capillary discharge sources have the potential advantages in that they can be simpler in design than both synchrotrons and LPS""s, and that they are far more cost effective. Klosner et al., xe2x80x9cIntense plasma discharge source at 13.5 nm for extreme-ultraviolet lithography,xe2x80x9d Opt. Lett. 22, 34 (1997), reported an intense lithium discharge plasma source created within a lithium hydride (LiH) capillary in which doubly ionized lithium is the radiating species. The source generated narrow-band EUV emission at 13.5 nm from the 2-1 transition in the hydrogen-like lithium ions. However, the source suffered from a short lifetime (approximately 25-50 shots) owing to breakage of the LiH capillary.
Another source is the pulsed capillary discharge source described in Silfvast, U.S. Pat. No. 5,499,282, which promised to be significantly less expensive and far more efficient than the laser plasma source. However, the discharge source also ejects debris that is eroded from the capillary bore and electrodes. An improved version of the capillary discharge source covering operating conditions for the pulsed capillary discharge lamp that purportedly mitigated against capillary bore erosion is described in Silfvast, U.S. Pat. No. 6,031,241.
Debris generation remains one of the most significant impediments to the successful development of the capillary plasma discharge sources in photolithography. The particles are ejected from the surfaces of the capillary and electrode due to ablation caused by the absorption of intense pulses of electrical energy. These particles are generally small (less than one micron) and have very large velocities (greater than 100 m/s).
FIG. 8 is a cross sectional view of an electric capillary discharge source which has a longitudinal arrangement whereby the capillary 1 and electrodes 2 and 5 consist of cylindrical disks with an on-axis opening. A high voltage pulse is applied to the electrodes 2 and 5 which generates a plasma 6 leading to short wavelength emission from the plasma. The radiation is emitted along this axis and collected and directed for some purpose such as extreme-ultraviolet lithography. This design suffers from some drawbacks; the main concern with this geometry is that debris is generated by the interaction of the plasma with the walls of the capillary and the electrodes. This debris is also ejected along the axis or bore in the same direction that useful light is collected. Mirrors and other light collecting optics are directly in-line with the debris and thus quickly become coated with the ablated material. This has the effect of reducing the effectiveness and lifetime of the light collecting optics.
Debris is generated when the high voltage and high current pulse heats the plasma and causes electrons and ions to stream through the bore and impact the electrodes. Sputtering of the electrode material is an inherent limitation of this type of longitudinal design. Thus, the electrodes themselves are a significant source of the debris composition.
The present invention is based in part on the recognition that debris generation from an EUV electric discharge plasma source can be significantly reduced or essentially eliminated by encasing the electrodes with dielectric or electrically insulating material so that the electrodes are shielded from the plasma. Additionally, the radiation escapes or is collected through the side of the device or transversely via one or more of a radiation exit, and away from any contact with the electrodes.
In one aspect, the invention is directed to an extreme ultraviolet and soft x-ray radiation electric discharge plasma source that includes:
(a) a body, which is made of an electrically insulating material, that defines a capillary bore that has a proximal end and a distal end and that defines at least one radiation exit;
(b) a first electrode that defines a first channel that has a first inlet end that is connected to a source of gas and a first outlet end that is in communication with the capillary bore, wherein the first electrode is positioned at the distal end of the capillary bore;
(c) a second electrode that defines a second channel that has a second inlet end that is in communication with the capillary bore and a second outlet end, wherein the second electrode is positioned at the proximal end of the capillary bore; and
(d) a source of electric potential that is connected across the first and second electrodes, wherein radiation generated within the capillary bore is emitted through the at least one radiation exit and wherein the first electrode and second electrode are shielded from the emitted radiation.
In a preferred embodiment, the discharge plasma source has one radiation exit which is located at or near the middle of the capillary bore as measured along its length. In another embodiment, the capillary bore has two radiation exits that are located on the side of the body and that are substantially opposite each other. In a third embodiment, the discharge plasma source defines an open gap that separates the body into two units so that radiation radiates in a full 360 degrees azimuthal direction or some fraction thereof.